1. Field of the Invention
The present invention relates to a projection exposure system used for accomplishing the lithographic processes for manufacturing, for example, semiconductor integrated circuits, liquid crystal display devices, etc., and particularly to such a projection exposure system with a mechanism for efficiently detecting and correcting magnification and other factors of a projection optical system used in the projection exposure system. The present invention is also applicable to a step-and-scan type of scanning projection exposure system in which patterns of a mask are projected for exposure onto one shot-field after another on a photosensitive substrate after each shot-field is moved to a scanning starting position in a stepping fashion.
2. Description of the Related Art
In the art of the projection exposure systems used for accomplishing the lithographic processes for manufacturing, for example, semiconductor integrated circuits, liquid crystal display devices, etc., it is desired to keep constant at all times the image formation characteristics of a projection optical system used in the projection exposure system with high accuracy in order to ensure that the fine patterns of a reticle (or a photomask) can be projected onto a resist-coated wafer (or a glass plate, etc.) with high resolution, and/or that the patterns of a reticle can be projected onto the wafer while the next patterns in sequence will be aligned on the previous patterns formed on the wafer with precise registration.
However, it is often impossible to keep constant the image formation characteristics of the projection optical system, due to the changes in the environmental factors of the projection optical system such as the atmospheric pressure and temperature, the changes in the shapes of the projection image of the patterns of the reticle and/or the projection optical system which may be induced by the heat generated by the absorption of the illumination light, or the changes in the shapes of the patterns on the reticle which may be induced when so-called phase-shift masks are used. On the other hand, in recent years, various illumination methods have been devised in order to satisfy the strict requirements for the finer patterns of semiconductor devices and the like. For example, there have been proposed illumination methods including Annular Illumination method (Japanese published patent application No. Sho-61-91662) in which reticle patterns are illuminated with an illumination light beam which has an annular light intensity distribution defined in the pupil plane or in a plane near the pupil plane of the illumination optical system, and Modified Light Source method or Oblique Incidence Illumination method (Japanese published patent application Nos. Hei-4-101148 and Hei-4-408096) in which reticle patterns are illuminated with an illumination light beam incident on the reticle obliquely at a predetermined angle relative to the surface of the reticle, and in which the illumination light beam has a light intensity distribution in the pupil plane or in a plane near the pupil plane of the illumination optical system so defined that at least one maximum is present at a point eccentric from the optical axis of the illumination optical system a predetermined distance. When an illumination method used in a projection exposure system is changed from a conventional one to Annular Illumination method or Modified Light Source method, the image formation characteristics often change due to a change in the illumination conditions.
In an attempt to overcome these problems, there have been proposed and practiced various methods of correcting image formation characteristics. Several methods are proposed particularly for making corrections for the variations in the image formation characteristics of the projection optical system which are induced by the absorption of the exposure light energy. For example, U.S. Pat. No. 4,666,273 proposes one of such methods, in which the energy (heat) which is accumulated in the projection optical system when it is illuminated with the exposure light beam (for example, a light beam emitted from a KrF excimer laser having I-line spectrum) is continuously calculated, the change in the image formation characteristics to be induced by the accumulated energy is predicted, and fine adjustment of the image formation characteristics is performed through a certain correction mechanism.
The projection magnification, which is one of the basic characteristics of the projection optical system, tends to change due to, for example, the heat generated by the illumination with the exposure light beam and the changes in the atmospheric pressure and other factors, as described. In an attempt to make the variations in the magnification ratio as small as possible, various techniques have been practiced including one in which spaces between the lenses within the projection optical system are hermetically sealed and the pressures in the spaces are adjusted, and one in which some of the lenses of the projection optical system are moved along the optical axis for adjustment.
In recent years, with the advance in scaling down patterns of semiconductor integrated circuits and the like, the importance of the variations in distortion (including so-called pin-cushion distortion and barrel-form distortion) of the projection optical system have increased. Thus, there have been proposed mechanisms for making corrections for the variations in the distortion, including one for moving the reticle along the optical axis of the projection optical system, one for moving some of the lenses of the projection optical system along its optical axis, and one for changing the wavelength of the light emitted from the exposure light source (such as a laser).
More recently, there have been proposed various scanning projection exposure systems (step-and-scan projection exposure systems) in which both the reticle and the wafer are moved relative to the projection optical system for scanning, so as to meet the needs for exposing a larger area with constant image formation characteristics.
A prototype of such scanning projection exposure systems is a "reflection projection aligner", in which a reflecting projection optical system with the magnification 1:1 (equal ratio) is used, a reticle stage for holding an equal ratio reticle (or "mask" in its narrower definition) and a wafer stage for holding a wafer are fixedly connected to a common moving column, and the reticle and the wafer are moved as an integral unit and thus at the same velocity for scanning exposure. The equal ratio reflecting projection optical system has no refracting element and is never subject to a chromatic aberration in a wide wavelength range of the exposure light. Thus, more than one line spectra from a light source (for example, G-line and H-line spectra from a mercury vapor lamp) can be used simultaneously so as to enhance the exposure intensity, thereby enabling a high speed scanning exposure. The reflecting projection optical system has the point where both the astigmatism in the sagittal plane and that in the meridional plane are to be zero, the location of which point is however limited to be within a region near a certain image height position distant from the optical axis of the reflecting projection optical system a certain distance, and thus, the shape of the exposure light beam is defined to be a portion of a narrow annular ring, that is, so-called arc-slit-shaped.
In the equal ratio scanning projection exposure systems with equal ratio reflecting projection optical system, two types of projection optical systems may be used; one projects onto the wafer such image of the reticle patterns that is not a mirror image of the actual reticle patterns, and the other projects onto the wafer such image of the reticle patterns that is a mirror image of the actual reticle patterns. When the former type of projection optical system is used, the reticle and the wafer are fixedly held on one moving column and in alignment with each other, and the moving column is moved linearly in the direction transverse to the longitudinal direction of the arc-slit-shaped illumination area for scanning. When the latter type of projection optical system is used, separate reticle stage and wafer stage are required and moved in opposite directions at the same velocity.
There is also known another type of scanning projection exposure techniques in which a projection optical system having refracting elements and of the magnification not 1:1 (equal ratio) is used, and the reticle stage and the wafer stage are moved for scanning at velocities which are in ratio equal to the magnification. This type of scanning projection exposure system may include a projection optical system using both reflecting refracting elements, or alternatively, a projection optical system using refracting elements only. Japanese published patent application No. Sho-63-163319 discloses a magnification projection optical system using both reflecting and refracting elements.
In this technique, the maximum effective diameter of the exposure field can be utilized by illuminating the reticle with a slit-shaped illumination light beam. Further, exposure field may be advantageously extended in the scanning direction without any limitation imposed from the construction of the projection optical system. Moreover, high uniformity in the illumination and high accuracy with respect to the distortion may be relatively easily achieved because only a part of the field of the projection optical system is utilized.
Any and all of these projection exposure techniques require some method of effectively predicting or detecting the variations in the magnification and or the distortion of the projection optical system. There have been proposed two types of such methods; indirect detection method and direct detection method. In an indirect detection method, the illumination energy irradiated onto the projection optical system is measured by an illumination sensor and/or any variations in the atmospheric pressure are measured by an atmospheric pressure sensor, and then the measurements are used to predict an appropriate amount of correction. In a direct detection method, the variations in the projection magnification and/or the distortion of the projection optical system are directly measured by means of a suitable measuring technique. In one example of such measuring technique, the relative position shift between a first position mark formed on the reticle and a second position mark formed on the wafer or on any other member equivalent to the wafer for the purpose is measured, and the measurement is used to estimate the projection magnification.
However, in any of the existing projection exposure systems, when an indirect detection method is used, a substantial error tends to occur because of the individual difference of the projection optical systems in the characteristics based on which correction amount is calculated. In order to avoid such error, the characteristics of the projection optical systems of all the projection exposure systems must have been previously measured, which is however very time-consuming.
Further, with the advance of the scale down of the patterns of the semiconductor integrated circuits and the like, such projection exposure systems have been designed that are capable of selecting one from a plurality of illumination methods including Annular illumination method and Modified Light Source method. The selection can be switched form one to another depending on patterns to be transferred onto the wafer. However, the characteristics of the projection optical system is affected by the switching of the illumination method. Therefore, this method tends to increase the error in the image formation characteristics and more time is required to measure the image forming characteristics.
On the other hand, when a direct detection method is used, a reference mask must be mounted or demounted from the projection exposure system every time the measurement is made. The work of mounting and demounting of the reference mask as well as the care of the reference mask are time-consuming, and further, each time the reference mask is mounted on the projection exposure system, there must be some variation present in the positioning of the reference mask, which leads to image formation errors.
Moreover, in any of the existing detection method, a reticle having a positioning marks formed thereon for measurement (i.e., a reference mask) must be mounted and demounted on and from the projection exposure system every time the measurement is made. The work of mounting and demounting the reference mask as well as the care of the reference mask are troublesome, and further, each time the mask is mounted on the projection exposure system, there must be some variation present in the positioning of the reference mask, which leads to measurement errors.
Although the scanning projection exposure systems have a specific feature that the reticle and the wafer are separately moved for scanning, any and all the existing detection methods are suited only for the projection exposure technique in which the reticle and the wafer are moved together as an integral unit for scanning. Thus, a detection method suitable for the scanning projection exposure systems are highly desired.